The present invention relates to an apparatus for purifying and controlling exhaust gases discharged from internal combustion engine or the like of automobiles, etc., and particularly to an apparatus for purifying and controlling the exhaust gases discharged from an automobile equipped with an internal combustion engine which can be driven at a lean burn.
The exhaust gases discharged from the internal combustion engine of automobile, etc. contain oxygen monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), etc., and these gases act as air-polluting substances to cause various problems. A great endeavor has been made so far for decreasing the discharge of these gases. Thus, it has been studied to decrease the generation of these gases by improving the combustion method in internal combustion engines. In addition, a method for purifying the discharged exhaust gas by the use of a catalyst or the like has also been studied to achieve considerable results. Today, in the field of gasoline engines, predominant is the use of a ternary catalyst composed of Pt and Rh as main components with which the exhaust gas can be made harmless through a simultaneous oxidation of HC and CO and reduction of NOx.
Said ternary catalyst, however, can effectively be used, in its nature, only for exhaust gases formed from a combustion in which the air/fuel ratio is in neighborhood of the theoretical air/fuel ratio. Although the air/fuel ratio varies with the conditions of driving the automobile, the range of the variation has hitherto been regulated so as to fall in the neighborhood of the theoretical air/fuel ratio as a rule (theoretical air/fuel ratio=A (weight of air)/F (weight of fuel); in case of gasoline, A/F=ca. 14.7; in this specification, the theoretical air/fuel ratio A/F is fixed at 14.7, even though it may vary depending on the kind of fuel). However, if an engine can be driven at an air/fuel ratio leaner than the theoretical air/fuel ratio, specific fuel consumption will be improved. Accordingly, the technique of lean burn combustion has been studied, and in many of the today""s automobiles, the internal combustion engine is driven in the lean burn zone where air/fuel ratio is 18 or above. Although the existing ternary catalyst can purify the HC and CO in the lean-burn exhaust gas oxidatively as has been mentioned above, it cannot reductively purify the NOx effectively. Accordingly, a technique for purifying exhaust gases capable of coping with lean burn driving must be developed in order to apply the lean burn technique to large-sized automobiles and prolong the lean burn combustion time (expansion of the driving condition zone to which lean burn technique can be applied). For such a reason, a technique for purifying exhaust gases capable of coping with the lean burn, namely a technique for purifying HC, NO and NOx and especially NOx from an exhaust gas having a high oxygen content, is being studied energetically.
In Japanese Patent 2586739 (U.S. Pat. No. 5,437,153), there is proposed an apparatus for purifying exhaust gases provided with an NOx absorbent and an NOx releasing means in the exhaust pipe of an internal combustion engine, said apparatus acting so as to estimate the quantity of NOx absorbed on the NOx absorbent and, when the estimated quantity of absorbed NOx has exceeded a previously determined allowable limit, to lower the oxygen concentration in the exhaust gas flowing into the NOx absorbent and thereby releases NOx from the NOx absorbent. On the other hand, a method for making NOx harmless by NOx adsorption instead of NOx absorption is proposed in JP-A 10-212933 (WO97/47864). According to the method of JP-A 10-212933, there is proposed a function of directly reducing NOx to harmless N2 with an adsorbent highly reactive with NOx.
According to the prior art, the captured NOx is made harmless by changing the air/fuel ratio to the theoretical air/fuel ratio or a rich air/fuel ratio. Although this change-over of air/fuel ratio is carried out by estimating the quantity of adsorbed NOx, no consideration is made there about the function of NOx adsorption catalyst that can make NOx harmless by its direct reduction into N2.
The present invention relates to an apparatus for purifying and controlling exhaust gases which uses an NOx adsorption catalyst, and an object of the present invention is to provide an apparatus for purifying and controlling exhaust gases with which the NOx adsorbed on the NOx adsorption catalyst can be reduced in a proper timing with high accuracy.
The present invention provides an exhaust gas purification apparatus for use in a lean burn internal combustion engine comprising:
an exhaust gas duct connected to an engine through which an exhaust gas containing NOx gas passes,
an NOx adsorption catalyst disposed in said exhaust gas duct, said adsorption catalyst being able to chemically adsorb a part of NOx in the exhaust gas in a lean burn state wherein the amount of an oxidizing agent is larger than the amount of a reducing agent in a redox stoichiometric relation of components in the exhaust gas and to catalytically reduce NOx adsorbed on the catalyst in a state wherein the amount of the reducing agent is equal to or larger than the amount of the oxidizing agent, and
a control apparatus which decides a timing for producing a state wherein the amount of the reducing agent is equal to or larger than the amount of the oxidizing agent, and carries out reduction treatment of the NOx adsorbed on the adsorption catalyst by catalytical reaction with the reducing agent to form harmless N2, when the NOx adsorbed amount reaches a predetermined amount, which is calculated from the NOx amount obtained by subtracting the amount of NOx directly reduced to N2 during the lean burn state from the amount of NOx discharged from the internal combustion engine.
The present invention also provides an exhaust gas purification apparatus for use in a lean burn internal combustion engine comprising:
an exhaust gas duct connected to an engine through which an exhaust gas containing NOx gas passes,
an NOx adsorption catalyst disposed in said exhaust gas duct, said adsorption catalyst containing as apart of catalyst components at least one element selected from the group consisting of potassium, sodium, magnesium, strontium and calcium, and being able to chemically adsorb a part of NOx in the exhaust gas in a lean burn state wherein the amount of an oxidizing agent is larger than the amount of a reducing agent in a redox stoichiometric relation of components in the exhaust gas and to catalytically reduce NOx adsorbed on the catalyst in a state wherein the amount of the reducing agent is equal to or larger than the amount of the oxidizing agent, and
a control apparatus which decides a timing for producing a state wherein the amount of the reducing agent is equal to or larger than the amount of the oxidizing agent, and carries out reduction treatment of the NOx adsorbed on the adsorbing catalyst by catalytical reaction with the reducing agent to form harmless N2, when the NOx adsorbed amount reaches a predetermined amount, which is calculated from the NOx amount obtained by subtracting the amount of NOx directly reduced to N2 during the lean burn state from the amount of NOx discharged from the internal combustion engine.